1. Field of the Invention
The present invention relates to a method for adjusting the frequency of a piezoelectric resonator by irradiating an ion beam.
2. Description of the Related Art
Conventionally, as a method for adjusting the frequency of a piezoelectric resonator by irradiating an ion beam, as disclosed in Japanese Unexamined Patent Application Publication No. 63-151103, there is known a frequency adjustment method. In this method, a plurality of piezoelectric resonators is arranged on a cathode and a positive gas ion particle emitted from an anode is moved to the cathode. With this arrangement, etching is uniformly performed on the surfaces of the electrodes of the piezoelectric resonators to make rough adjustments. After the rough adjustments have been completed, an ion beam from an ion gun is irradiated onto each of the piezoelectric resonators to make fine adjustments.
In this case, during the fine adjustment phase, the ion-beam etching is performed while measuring the frequency of each element. Thus, the frequency can set to be close to a target frequency with high accuracy.
In this method, however, the frequency adjustment is performed by irradiating an ion beam onto the piezoelectric resonator when it is in the stage of an almost completed product. Consequently, when adjusting the frequencies of many piezoelectric resonators, it requires a great deal of time to individually position and retain the elements. Thus, there is a problem in terms of equipment performances.
Additionally, in the rough adjustment phase, since the ion beam is uniformly irradiated onto the plurality of piezoelectric resonators, variations in the frequencies of the elements are inevitably generated.
Furthermore, during the fine adjustment phase, after the irradiation of the ion beam is performed once, the frequency is measured. Sequentially, the ion beam irradiation is repeated. As a result, in order to make the frequency close to a target value, it is necessary to repeat the ion-beam irradiation and the frequency measurement many times. Thus, there is a problem in that it takes a great deal of time to make the frequency adjustment.
In order to overcome the problems described above, preferred embodiments of the present invention provide a method for adjusting the frequency of a piezoelectric resonator, in which the frequency of each of a plurality of elements disposed on a piezoelectric substrate can be adjusted before the completion of the resonator, with high accuracy in a relatively short time.
According to a preferred embodiment of the present invention, there is provided a method for adjusting the frequency of a piezoelectric resonator, in which the frequency of each of elements formed in a piezoelectric substrate is adjusted by irradiating an ion beam onto a plurality of electrodes provided on the piezoelectric substrate to etch the electrodes. The method includes the steps of determining a correlation between time for ion-beam irradiation and the amount of frequency changes, measuring the frequency of each of the elements disposed on the piezoelectric substrate, determining the amount of frequency adjustment for each element based on a difference between the measured frequency value and a target value, determining an ion-beam irradiation time for each element based on the determined amount of frequency adjustment by using the correlation, and irradiating an ion beam onto each element during the determined irradiation time.
First, the correlation between the time for irradiating an ion beam and the amount of frequency changes is determined. In this step, before the frequency of the piezoelectric resonator is actually adjusted, an ion beam is irradiated onto the electrodes. After this, the correlation between the ion-beam irradiation time and the amount of frequency changes is determined and stored as data. The stored data varies with the characteristics of an ion gun and the etching characteristics of the electrodes with respect to the ion beam.
Secondly, a frequency signal is input to each of the electrodes formed in insular forms on the piezoelectric substrate to measure the resonance frequency or anti-resonance frequency of each element defined by the mutually opposing electrodes provided on the upper and lower surfaces of the piezoelectric substrate and the piezoelectric substrate positioned therebetween. Then, based on the difference between the measured frequency value and the target value, the amount of frequency adjustment of each of the elements is determined. When the electrodes are arranged in a matrix on the piezoelectric substrate, it is preferable to store the data by correlating the positions of the elements to the amounts of frequency adjustments.
Thirdly, the ion-beam irradiation time for each element is determined from the determined amount of frequency adjustment by using the correlation between the irradiation time and the amount of frequency changes. Then, an ion beam is irradiated on each element during the obtained irradiation time. With this method, the frequencies of the plurality of elements disposed on the piezoelectric substrate can be individually adjusted. As a result, frequency variations in the piezoelectric substrate are minimized.
Moreover, since the correlation that is determined in advance is used to obtain the ion-beam irradiation time for each element and the ion beam is irradiated during the obtained irradiation time, the frequency of each element can be set to be close to a target value with a one-shot or one-time only irradiation. In other words, unlike the conventional method, it is unnecessary to repeat ion-beam irradiation and frequency measurement. Accordingly, in preferred embodiments of the present invention, the frequency value of each element can be adjusted to the target value with high accuracy in a short time.
When an ion beam from the ion gun is irradiated onto the plurality of electrodes, according to the irradiated positions on the electrodes, the current density varies. Thus, in order to eliminate frequency variations due to positional variations, preferably, the current density distribution of the ion beam is measured in advance to correct the ion-beam irradiation time obtained from the correlation according to the current density. With this arrangement, the amount of etching performed in a wide range by the ion beam is stabilized, thereby greatly increasing frequency concentration.
In addition, when the ion beam is continuously irradiated, the current density reduces with the passage of time since the grid of an ion-gun is worn out. Changes in the current density occurring with the passage of time increase frequency variations in the piezoelectric substrate. In this case, as mentioned above, the irradiation time can be corrected. On the other hand, since the irradiation time is prolonged, equipment performance is deteriorated. Thus, preferably, after measuring the current density of the ion beam, the measured current density is compared with a maximum and a minimum. When the current density is over the maximum and below the minimum, the intensity of the ion beam may be adjusted. In other words, when the current density is below the minimum, the intensity of the ion gun, that is, by increasing the value of a current discharged from the ion gun, the absolute value of the current density is corrected to be within a range of the maximum to the minimum. As a result, the frequency concentration can be greatly improved. In contrast, when the current density is over the maximum, the value of the current discharged from the ion gun will be decreased.
The current density of the ion beam may be measured in each piezoelectric substrate or in each product lot, in advance, afterwards, or at intervals of certain periods of time.
In various preferred embodiments of the present invention, it is preferable to individually vary the ion-beam irradiation time for each of the elements in the piezoelectric substrate. Thus, preferably, whereas the ion beam is continuously irradiated, there are disposed shutters for shielding and releasing the ion beam to adjust the irradiation time by opening and closing the shutters disposed in a manner adapted to each element.
With the shutters, while continuously irradiating the ion beam, the irradiation time for each element can be freely varied. Accordingly, the frequency adjustment can be performed with high accuracy.